Industrial electrolysis, particularly electrolysis of mainly inorganic acids, is being conducted in an extremely wide range of fields such as electrolytic refining of metals, electroplating, electrolytic syntheses of organic substances and inorganic substances, etc. Although lead or lead alloy electrodes, platinum-plated titanium electrodes, carbon electrodes, and the like have been proposed as electrodes, especially anodes, for use in such electrolytic processes, each of these electrodes has certain drawbacks, and hence, none of them have come into practical use in a wide range of electrolytic applications. For example, lead electrodes having on the surface thereof a layer of lead dioxide which is relatively stable and has good electrical conductivity, have the drawbacks that even this lead dioxide dissolves away under ordinary electrolytic conditions at a rate of several milligrams per amperehour and the electrode shows a large overvoltage. Platinum-plated titanium electrodes have a short life for their high price. Further, carbon electrodes have the drawbacks that where the anodic reaction is an oxygen-evolving reaction, the carbon electrode reacts with the evolved oxygen to consume itself as carbon dioxide, and the electrode has poor electrical conductivity.
In order to eliminate these drawbacks of conventional electrodes, a dimensionally stable electrode (DSE) has been proposed and is being used extensively.
The DSE functions as a long-life electrode having exceptionally good chemical stability so long as it employs a valve metal such as titanium as the substrate and is used as an anode, because the surface of the substrate is passivated. However, when the DSE is used as a cathode and undergoes a cathodic polarization, the substrate turns into a hydride through reaction with evolved hydrogen and, as a result, the substrate itself becomes brittle or the surface covering peels off due to corrosion of the substrate, leading to a considerably shortened electrode life. This is a serious drawback when the DSE is used in electrolytic processes in which the current flow is reversed.
In addition, the DSE has another problem in that if it is used in an electrolyte solution containing fluorine or fluoride ions even in a slight amount, the substrate comprising titanium or a titanium alloy suffers corrosion, shortening the electrode life considerably even when the electrode is used as an anode. For example, if the DSE is used in an electrolyte solution containing fluorine in an amount as slight as about from 3 to 5 ppm, the electrode life is, at the most, one-tenth the ordinary life of the electrode. Thus, this problem constitutes a serious obstacle to possible applications of the DSE to various electrolytic fields other than soda-producing electrolysis for which the electrode can be used completely satisfactorily.